Description:
------------
Several MBstring functions are much slower when called with encoding parameter than without encoding parameter.
Functions that exhibit this behavior are mb_strlen, mb_substr, mb_strpos, mb_strrpos.

For more information see:
https://stackoverflow.com/questions/45028018/a/45107408


Test script:
---------------
mb_internal_encoding("UTF-8");
echo "without encoding parameter:\n";

$a = microtime(true);
for($i=0; $i<100000; $i++){
    $n = mb_strlen("あえいおう");
}
$a = microtime(true)-$a;
echo "- mb_strlen: ".number_format($a*1000)." ms\n";

echo "\nwith encoding parameter:\n";

$b = microtime(true);
for($i=0; $i<100000; $i++){
    $n = mb_strlen("あえいおう", "UTF-8");
}
$b = microtime(true)-$b;
echo "- mb_strlen: ".number_format($b*1000)." ms (".number_format(($b-$a)*100/$a)."% slower)\n";


Expected result:
----------------
Because the character encoding is UTF-8 in both cases, I expected to see similar performance.
In other words, 0% slower or close to it.


Actual result:
--------------
When called with encoding parameter, performance drops greatly.
Example output from the test script:

without encoding parameter:
- mb_strlen: 14 ms

with encoding parameter:
- mb_strlen: 585 ms (4,186% slower)